Since polyethylene terephthalate resins have excellent mechanical strength, chemical stability, gas barrier properties, hygienic properties, plus relatively low cost and light weight, they have been used for the bottles for various types of drinks, and the increase of their production in recent years is phenomenal. The bottles made of these polyethylene terephthalate resins are produced, for instance, by a process which comprises injection molding a bottomed tubular preform, reheating this preform to soften it, and subjecting it to drawing blow molding. This process involves the problem that acetaldehyde is formed as a by-product in the molten resin during injection molding of the preform, and in use of the moldings as bottles, the by-product acetaldehyde may get into the contents of the bottles to affect their original flavor.
As a solution to the above problem, there have been proposed, for instance, a method in which the moisture content in the polyethylene terephthalate resin is adjusted to 60 to 500 ppm by weight before it is subjected to melt molding (see, for instance, Patent Document 1), and a method in which, as an improvement from the aspect of polycondensation reaction catalyst, the polyethylene terephthalate resin polycondensed in the presence of a polycondensation reaction catalyst is brought into contact with a phosphorus-containing solution for treatment therewith, the said polycondensation reaction catalyst comprising a titanium compound and a compound of a metal element such as magnesium in a specific ratio (see, for instance, Patent Document 2). There is also known a polyethylene terephthalate resin producing process in which, in order to suppress the formation of acetaldehyde as a by-product during the molding operation, the starting material is polycondensed in the presence of both a polycondensation reaction catalyst and a phosphorus compound in a specific ratio, the said polycondensation reaction catalyst comprising a titanium compound and a compound of a metal element such as magnesium in a specific ratio (see, for instance, Patent Document 3).
In the method described in Patent Document 1, however, the suppressive effect against the formation of by-product acetaldehyde during the melt molding operation is not necessarily satisfactory, and further, when the molding operation is continued for a long time, the moisture content in the resin may change to cause a variation of acetaldehyde content in the moldings, resulting in a reduction of yield of the products with an acetaldehyde content in the acceptable range, so that means for preventing the drop of yield has been sought. The methods described in Patent Document 2 and Patent Document 3 are also unable to provide a satisfactory inhibitory effect against the formation of acetaldedye in the molding operation and fall short of meeting the market demand. Thus, the development of techniques enabling further suppression of the formation of by-product acetaldehyde in the production of polyethylene terephthalate resin moldings has been desired.    Patent document 1: Japanese Patent Application Laid-Open (KOKAI) No. 7-205257    Patent Document 2: Japanese Patent Application Laid-Open (KOKAI) No. 2001-81161    Patent Document 3: Japanese Patent Application Laid-Open (KOKAI) No. 2002-226563